Electronic components are mounted on printed circuit boards mostly by soldering. Common soldering includes, prior to application of a solder, removal with a flux of the oxide film on a metal surface to be jointed, and cleaning, or alternatively is implemented using a solder paste containing solder powder in mixture with a flux.
Soldering fluxes are required to be highly insulative, noncorrosive, stable for a long time, and noncausative of material property change of other parts, for maintaining good properties and reliability of the resulting products. On the other hand, for good workability, soldering fluxes are also required not to generate toxic gases, to give good solder ability by removing oxides from and covering the metal surface and by lowering the surface tension of molten solder, not to be tacky, and to be easy to remove by cleaning.
A flux usually contains a resin, an activator, a solvent, and other additives, and is usually used by applying the flux to a surface of a metal substrate, or by soaking the substrate in the flux. A solder paste is a viscous paste made of a solid or liquid flux and solder powder kneaded together. The flux in a solder paste usually contains a rosin compound, a thixotropic agent, an activator, a solvent, and a dispersion stabilizer. As the base resin for the flux, a rosin compound is used, such as natural rosin, polymerized rosin, hydrogenated rosin, or disproportionated rosin. As the activator for the flux, an organic acid is used, such as adipic, sebacic, or citric acid.
The carboxyl groups in the rosin compound and in the organic acid as an activator in the flux, function to remove metal oxides on a metal surface to effectively improve solder ability. However, such carboxyl groups form a post-solder residue, which adversely affects the reliability after soldering due to its ionic and mechanical effects. In a solder paste containing a flux, the carboxyl groups also adversely affect the storage stability of the solder paste per se. Specifically, the flux component in a recent lead-free solder necessarily contains a relatively strong activator in a large amount for compensating for inferior wettability of the lead-free solder. However, such increased activity or amount of the activator impairs the reliability of the solder joint, as well as the storage stability of the solder paste. Thus wettability required for the flux and the solder paste, and other necessary properties such as reliability and storage stability are conflicting with each other, and all of these requirements are hard to be fulfilled at the same time. Further, the kind and amount of the activator need to be strictly limited.
As a flux remover, fluorinated or chlorinated solvents have been in use, but their use is being limited for their impact on the environment. Hydrocarbon and alcohol solvents have also been in use, with problems in toxicity and inflammability. In this sense, water is the most preferable defluxer, but no flux or solder paste has been available that is sufficiently cleaned with water. When a flux is to be cleaned with water, the non-volatile components of the flux must be water-cleanable, and even the volatile solvent is preferably water-cleanable, since a part of the solvent may be left over through the reflow. In general, rosin compounds used as a base resin for the flux component is hardly water-cleanable, and thus not usable in a flux to be cleaned with water.
On the other hand, low-residue, no-clean fluxes with a low solid content are commercially available. However, reliability of such no-clean fluxes is not yet sufficient when a resin molding or wire bonding process is to follow. Commercially available no-clean solder pastes are designed to have a lower solid content or no halide content, yet inevitably leave post-solder residues derived from the resin component such as rosin or the activator component. The residue from the activator particularly has serious impact on the reliability. On the contrary, the residue from the resin, such as rosin, does not have a serious effect on the soldering of most printed circuit boards for consumer electric appliances and some kind of vehicle on-board printed circuit boards. However, in soldering HICs, even a little residue will give rise to terminal leakage or circuit corrosion of flip chips, leading to problems in the reliability in the following wire bonding step.
For solving such problems, JP-2-290693-A and JP-2-25291-A propose a soldering method that leaves no residue, including reflowing a solder paste with alcohol. The former discloses a paste containing solder powder mixed with alcohols, such as monohydric alcohols, polyhydric alcohols, or ethers, with a boiling point about 30° C. higher than the melting point of the solder. This solder paste may be a residueless material, but may not be expected to have good wettability under a non-reducing atmosphere or at an ordinary reflow temperature.
JP-2001-239395-A proposes a thermosetting soldering flux containing compound (A) having one or more blocked carboxyl groups in a molecule, and compound (B) having two or more reactive functional groups in a molecule capable of forming chemical bonds with carboxyl groups by heating. However, since this flux contains the requisite compound (B) having two or more reactive functional groups, the compound (A) having one or more blocked carboxyl groups is cured by thermal crosslinking. Thus this flux is a no-clean flux leaving a thermoset product, and thus is totally different from a flux requiring cleaning, or a no-clean, non-curing flux leaving substantially no residue, in its reaction, effects, and required properties.